As a representative example of a conventional liquid jetting apparatus, there is known an inkjet printer including an inkjet printing head for printing an image. Examples of other liquid jetting apparatuses can include an apparatus including a color-material jetting head which is used for manufacturing a color filter of a liquid crystal display or the like, an apparatus including an electrode-material (conductive paste) jetting head which is used for forming an electrode of an organic EL display, a field emission display (FED), or the like, an apparatus including a living-organic-material jetting head which is used for manufacturing a biochip, and an apparatus including a sample jetting head as a precision pipette.
In the inkjet printer which is the representative example of the liquid jetting apparatus, a carriage includes an inkjet printing head having a pressurizing unit which pressurizes a pressure generating chamber and a nozzle opening for ejecting pressurized ink as ink droplets. The inkjet printer has a structure that ink in an ink container continues to be supplied to the printing head through a flow path, thereby continuing a printing work. The ink container is constructed as a detachable cartridge which can be replaced by a user, for example, when the ink runs out.
Conventionally, as a management method for ink consumption of the ink cartridge, there are known a management method in which the number of ink droplets ejected from the printing head or the amount of ink absorbed by a maintenance is integrated in software to calculate the amount of consumed ink and a method of managing the time when a predetermined amount of ink is actually consumed by mounting an electrode for detecting a liquid level in the ink cartridge.
However, the management method, in which the number of ejected ink droplets or the amount of ink is integrated in software to calculate the amount of consumed ink, has the following problems. Some heads show a weight variation in ejected ink droplets. The weight variation in ink droplets dose not have an effect on display quality. However, in consideration of the time when errors in the amount of consumed ink due to the variation are accumulated, the ink cartridge is filled with the ink together with margin ink. Accordingly, depending upon the individual ink cartridges, ink is left as much as the margin ink.
On the other hand, in the method of managing the time when the ink is consumed with an electrode, the actual amount of ink can be detected, so that the amount of remaining ink can be managed with high reliability. However, there are drawbacks in that kinds of ink to be detected are limited and a seal structure for an electrode becomes complicated, because detection of an ink level relies on conductivity of ink. In addition, precious metal having excellent conductivity and corrosion resistance is generally used as a material of the electrode. Therefore, manufacturing cost of the ink cartridge runs up. Further, since two electrodes need to be mounted, the number of manufacturing processes is increased, thereby increasing the manufacturing cost.
Accordingly, a device developed for solving the above-mentioned problems is disclosed as a piezoelectric device (hereinafter, referred to as sensor unit) in Patent Document 1. When ink exists and does not exist inside a cavity facing a vibration plate on which a piezoelectric element is stacked, the sensor unit monitors the amount of ink remaining in an ink cartridge by the use of variation in resonance frequency of a residual vibration signal due to residual vibration (free vibration) of the vibration plate after compulsory vibration.
Patent Document 1: JP-A-2001-146030
In the technique disclosed in Patent Document 1, since the cavity of the sensor unit is allowed to directly face an ink storage chamber of the cartridge, the sensor unit can be easily affected by bubbles mixed into the ink or noises such as a wave motion of the ink, thereby decreasing accuracy of detection.